1. Field of the Invention
The present invention relates to a motor control device for compensating for backlash.
2. Description of the Related Art
A servomotor is connected to a feed axis of an industrial machine, such as a machine tool, or to an axis of an industrial robotic arm (mechanical movable unit). Rotary motion of the servomotor may be converted into linear motion of a table via a ball screw, or transmitted to the axis at a reduced rotational velocity via a speed reducer.
In the ball screw or the speed reducer, a mechanical element on the servomotor side such as a screw or gear and a corresponding mechanical element on the driven unit side which is driven by the servomotor are brought into engagement to transmit power therebetween. In some cases, a difference may exist between an engagement position where the mechanical elements engage with each other when the servomotor rotates in a positive direction and an engagement position where the mechanical elements engage with each other when the servomotor rotates in a negative direction. Such a difference is generally referred to as “backlash,” and could cause accuracy in positioning the driven unit by the servomotor to be impaired.
FIGS. 7A to 7C are drawings to explain backlash. FIGS. 7A to 7C illustrate a movable unit WA driven by a motor (not shown) and a driven unit WB driven by the movable unit WA. The movable unit WA is provided with protruding portions A1 and A2 at both ends thereof. The driven unit WB is provided with a protruding portion B at its center. When the movable unit WA moves rightward, an inner end of the protruding portion A1 of the movable unit WA on the left side engages with the protruding portion B of the driven unit WB (FIG. 7A). In this state, the movable unit WA and the driven unit WB together move to the right-hand side.
In order to change a movement direction of the driven unit WB from the rightward to the leftward, the movable unit WA is moved to the left-hand side (FIG. 7B). Then, when an inner end of the protruding portion A2 of the movable unit WA on the right side engages with the protruding portion B of the driven unit WB (FIG. 7C), the movable unit WA and the driven unit WB together move to the left-hand side.
As described above, in order to reverse a movement direction of the driven unit WB, the movable unit WA has to move by a predetermined movement amount C (see FIGS. 7A and 7C) corresponding to the backlash before the movable unit WA engages again with the driven unit WB after the engagement between the movable unit WA and the driven unit WB is released. The additional movement amount C necessary due to the backlash may result in reduced accuracy in positioning.
In order to prevent this, it has been known to generate a compensation amount for compensating backlash, which is to be added to a position command to a motor at the time of reversing in motion. JP-A-9-319418 discloses a related art in which a relationship between a feeding speed of the respective axes prior to a reverse in motion and a compensation amount corresponding to a movement amount before the motion is reversed is measured in advance to calculate a necessary compensate amount. JP-A-2000-250614 discloses a related art in which a compensation amount is adjusted, depending on time elapsed after the reverse in motion.
FIG. 8 is another drawing to explain backlash. FIG. 8 illustrates a state where neither the protruding portion A1 nor the protruding portion A2 of the movable unit WA engages with the protruding portion B. When the movable unit WA is then moved leftward, the movable unit WA moves over a distance C1 shorter than the distance C corresponding to the backlash to engage with the driven unit WB (also see FIG. 7C). This tends to occur when backlash is relatively large.
In such a case, if a constant compensation amount corresponding to the backlash is simply added to a position command to a motor, the resultant compensation for the backlash may become excessively large. Alternatively, in order to prevent a compensation amount from becoming excessively large, a compensation amount may also be set so as to be smaller than the backlash. However, in this case, there is a risk that a compensation amount may turn out to be too small to provide sufficient compensation to the backlash at the time of the reverse motion.
In this connection, Japanese Patent Application No. 2012-195098, which is an unpublished co-pending application filed by the applicant of the present application, relates to a motor control device for allowing an appropriate compensation amount to backlash to be obtained, irrespective of a positional relationship between a movable unit and a driven unit.
However, according to the related art proposed in Japanese Patent Application No. 2012-195098, it may be necessary to store a positional relationship between the movable unit and the driven unit when they are in engagement with each other. Such information can be stored by software; however, the stored information will be lost when electric power to the control device is shut off. The necessary information may be stored in a non-volatile memory, but this leads to an increase in cost. In addition, since the mechanical configuration and the mechanical property may be subject to change, it is preferable to obtain information necessary for backlash compensation at the time of activation of the control device.
Accordingly, there is a need for a motor control device for allowing optimal backlash compensation to be carried out even immediately after activation of the device.